The present invention relates to an improvement made to materials for acoustic and phonic absorption.
It relates more particularly to absorbent porous materials intended for buildings (wall coverings) and civil engineering installations (anti-noise devices), for which improvements in product performance are desired, in particular in terms of absorption coefficient, especially with respect to a specific frequency band.
Elements that absorb sound waves are often used for improving the acoustic conditions of noisy environments.
The sound waves, on encountering a material, depending on their properties, are generally partly reflected, diffracted, transmitted through the materials and absorbed.
By virtue of their properties, so-called acoustic absorbent materials limit the proportion of energy that is reflected.
Furthermore, the acoustic absorption of this material is also a function of the sensitivity, in terms of frequency response, of the receiving organ, which in the case of the human ear is between 20 and 20000 Hz. Generally it is difficult to find materials capable of treating the entire spectrum of the audible range, and in particular, the absorption of low frequencies, below 400 Hz, is very limited.
In the field of acoustic absorption, intended for industry or for domestic applications, materials are used which have the form of panels or of pulverulent products that are applied by spraying, and which are based on mineral glass wool, foam, fabric or alternatively of perforated rigid panels. In this last embodiment, reference may be made to Japanese patent JP10-175263, which describes an absorbent material formed of a sandwich consisting principally of a first layer of material provided with a plurality of holes that end in a plurality of cavities formed in a second layer backing onto the first. However, this absorbent material functions according to the Helmholtz principle, the energy of the sound waves being dissipated in the cavities.
These absorbent materials are selected according to their intended application (theatres or cinemas, conference halls, swimming baths, gymnasiums, dining halls, industrial environment etc.), their acoustic characteristics (frequency response), as well as their mechanical properties, fire resistance, heat absorption, rotproof properties etc.
These absorbent materials can be employed for exterior application as well as in a confined setting (interior application).
Thus, depending on the type of application, we are led to use for example mineral wools and certain foams with open porosity (a material is said to have open porosity when its pores are connected to one another and to the exterior) of low density or of higher density, and in this case they are used in applications for acoustic correction of ceilings or false ceilings (suspended ceilings) rather than in insulation of traditional walls.
Generally these products based on glass wool or foam possess suitable absorption properties, but they have three types of drawbacks:
they are usually relatively fragile and therefore of poor endurance, in particular on account of the low densities employed bearing in mind that we wish to obtain relatively high permeability and porosity, and this low mechanical strength makes them difficult to install;
their acoustic performance declines rapidly in the low frequency range (below 500 Hz), this effect being accentuated when these materials are made into thin panels;
installation of those materials as false ceilings makes it possible to obtain acceptable acoustic performance, but requires having a large air gap to guarantee the acoustic performance.
An acoustic absorption material formed from a porous inorganic substance and comprising a plurality of perforations positioned at an angle of less than 80xc2x0 relative to a specific dimension of the material is known from U.S. Pat. No. 4,113,053. Moreover, these perforations have a cross-section (square, circular or rectangular) that is of an area that can reach several tens of mm2.
In addition, a soundproofing panel consisting of fibres and having a plurality of perorations arranged on its surface at an angle between 10xc2x0 and 80xc2x0 is known from German Patent DE 33 39 701. Moreover, the cross-section of these perforations can be chosen arbitrarily.
The present invention therefore aims to overcome these drawbacks, by proposing improvements made to the acoustic materials that, endow them with good mechanical properties (mechanical strength), even when these materials have small thickness, while preserving their excellent acoustic absorption properties, especially in the low-frequency range.
To this end, the acoustic absorbent material, consisting of a porous substance with open porosity, having a plurality of perforations positioned at an angle xcex8 relative to a dimension consisting of the thickness of the material, thus imparting as additional porosity (Pa) to the material that is the object of the invention, is characterized in that the additional porosity (Pa) ranges from 10 to 30% and in that it has an acoustic absorption coefficient ranging from approx. 0.7 to 0.93 for low frequencies, in particular of the order of 300 Hz.